In recent years, nondestructive sensing techniques using an electromagnetic wave of millimeter-wave to terahertz (THz) wave frequencies (30 GHz to 30 THz) have been under development. An example of techniques now under development in application fields of the electromagnetic wave of such a frequency band is an imaging technique using a safe fluoroscopic apparatus alternative to an X-ray fluoroscope. In addition, examples of techniques now under development include a spectral technique for obtaining an absorption spectrum or complex dielectric constant of a material to examine the bonding state therein, a technique for analyzing biomolecules, and a technique for estimating a carrier concentration or mobility.
Up to now, a photoconductive device including antennas also serving as electrodes which are provided on a photoconductive film formed on a substrate is suitably used as an example of a THz wave generating means (see JP 10-104171 A). FIG. 8 shows a structural example of the photoconductive device. A substrate 130 has, for example, a silicon-on-sapphire structure treated with radiation. In the substrate 130, a silicon film which is a photoconductive material is formed on a sapphire substrate. An LT-GaAs film grown on a GaAs substrate at a low temperature is used as the photoconductive film in many cases. A dipole antenna 138 formed in a surface of the substrate includes a pair of dipole feeders 138a and 138b and a pair of dipole arm portions 139a and 139b. A light pulse is focused on a gap 133. When a voltage is applied across the gap 133, a THz pulse generates. When a light current is detected without the application of the voltage across the gap 133, the THz pulse can be detected. Here, the photoconductive device is a detector 132. The light current is detected by a current amplifier 134. A substrate lens 136 has a function of conducting coupling from a slab mode (substrate mode) of an electromagnetic wave confined in the substrate 130 to a radiation mode to free space, and a function of controlling a radiation angle of an electromagnetic wave propagation mode in space.
The above-mentioned structure is an example in which the electromagnetic wave is propagated through a space using a single photoconductive device. There has been also proposed a small functional device in which a semiconductor thin film serving as a photoconductive device and transmission paths for causing generated electromagnetic waves to propagate are integrated on a single substrate (see Applied Physics Letters, Vol. 80, No. 1, 7 Jan., 2002, pp. 154-156). FIG. 9 is a plan view showing the functional device. The functional device has a structure 164 in which a thin film composed of only a LT-GaAs epitaxial layer of the photoconductive device is transferred to a part of high-frequency transmission paths 165 and 163 formed on an Si substrate 160. In the structure 164, microstrip lines are formed on the substrate 160 so as to sandwich an insulator resin. A gap is produced in a part of the lines. The thin film of LT-GaAs is placed only under the gap. Driving is performed such that a laser beam is emitted from a surface side of the substrate 160 to the gap placed between metallic lines 161 and 165 through space propagation to propagate a generated THz electromagnetic wave to the lines. When a test sample 167 to be examined is applied to a filter region 166 having a resonant structure on the transmission paths, a change in propagation condition is detected from a portion 162 using an EO crystal. Therefore, the properties of the test sample 167 can be examined.
However, according to a method disclosed in Applied Physics Letters, Vol. 80, No. 1, 7 Jan. 2002, pp. 154-156, the amount of change in propagation condition of the terahertz-wave to the test sample is small. Therefore, a large amount of test sample is necessary. For the improvement of sensitivity, it is necessary to increase the strength of the electromagnetic wave. When a reference test sample is compared with a target test sample, it is necessary that data be obtained and stored for each step or that additional measurement be performed by sensing devices having different transmission paths. In this case, it is difficult to adjust a condition of measuring the reference test sample to the same condition as that of measuring the target test sample. Thus, it is difficult to accurately estimate the amount of change in the target test sample from the reference test sample.